Hydroprocessing or hydrotreatment to remove undesirable components from hydrocarbon feed streams is a well known method of catalytically treating such hydrocarbons to increase their commercial value. "Heavy" hydrocarbon liquid streams, and particularly crude oils, petroleum residua, tar sand bitumen, shale oil or liquified coal or reclaimed oil, generally contain product contaminants, such as sulfur, and/or nitrogen, metals and organo-metallic compounds which tend to deactivate catalyst particles during contact by the feed stream and hydrogen under hydroprocessing conditions. Such hydroprocessing conditions are normally in the range of 212.degree. F. to 1200.degree. F. (100.degree. to 650.degree. C.) at pressures of from 20 to 300 atmospheres. Generally such hydroprocessing is in the presence of catalyst containing group VI or VIII metals such as platinum, molybdenum, tungsten, nickel, cobalt, etc., in combination with various other metallic element particles of alumina, silica, magnesia and so forth having a high surface to volume ratio.
Because these reactions must be carried out by contact of a hydrogen-containing gas with the hydrocarbon feed stream at elevated temperatures and pressures, the major costs of such processing are essentially investment in vessels and associated furnaces, heat exchangers, pumps and tubing capable of such service and the replacement cost of catalyst contaminated in such service. Commercial hydroprocessing of relatively low cost feed stocks such as crude oils containing pollutant compounds, requires a flow rate on the order of a few thousand up to one hundred thousand barrels per day, with concurrent flow of hydrogen at up to 10,000 standard cubic feet per barrel of the liquid feed. Vessels capable of containing such a reaction process are accordingly cost-intensive both due to the need to contain and withstand metal embrittlement by the hydrogen, while carrying out the desired reactions, such as demetalation, denitrification, desulfurization, and cracking at elevated pressure and temperatures. For example, because of metallurgy and safety requirements, such vessels may cost on the order of $700.00 per cubic foot of catalyst capacity. Thus a vessel capable of handling 25,000 barrels per day of a hydrocarbon feed stream may run on the order of $4,000,000 to $5,000,000. Pumps, piping and valves for handling fluid streams containing hydrogen at such pressures and temperatures are also costly, because such pressure seals must remain hydrogen impervious over extended service periods of many months.
Further, hydroprocessing catalyst for such a reactor, which typically contains metals such as titanium, cobalt, nickel, tungsten, molybdenum, etc., may involve a catalyst inventory of 500,000 pounds and a cost $2 to $4/lb. Accordingly, for economic feasibility in commercial operations, the process must handle high flow rates and the vessel should be filled with as much catalyst as possible to maximize catalyst activity and run length. Additionally, the down-time for replacement or renewal of catalyst must be as short as possible. Further, the economics of the process will generally depend upon the versatility of the system to handle feed streams of varying amounts of contaminants such as sulfur, nitrogen, metals and/or organic-metallic compounds, such as those found in a wide variety of the more plentiful (and hence cheaper) crude oils, residua, or liquified coal, tar sand bitumen or shale oils, as well as used oils, and the like.
In prior systems for hydroprocessing, it is known to flow a liquid hydrocarbon feed and a hydrogen stream concurrently with the catalyst in a downward flow path. While this assures high packing density for the catalyst, without ebulation or expansion of the bed, such processes have a tendency to form local deposits of feed metals that plug the bed, particularly at the top catalyst bed in the vessel. Accordingly, in general, it is preferred to counterflow the catalyst and process fluid streams relative to each other. However, as noted above, when the process feed rates are high, the volume of catalyst that can be contained by the vessel may be as little as 10% of the original settled volume. At lower fluid velocities, catalyst volume may be up to about 80% to 90%, but useful reaction space for the process is still wasted and turbulence causes axial mixing of the catalyst which interferes with the desired plug flow movement. Therefore, it is a particular object of this invention to run a counterflow processing system where the catalyst and fluid velocity combinations limit bed expansion to less than 10%.
It is also known to use a series of individual vessels stacked one above the other, with fluid flow either concurrent or counterflow to catalyst. In such a process, catalyst moves by gravity from the upper vessel to a lower vessel by periodically shutting off, or closing, valves between the individual vessels. In a counterflow system, this permits removal of catalyst from the lowermost or first stage vessel, where the most contaminated, or raw, feed stock, originally contacts the catalyst. In this way, most of the major contaminating components in the hydrocarbon stream are removed before the hydrocarbon material reaches major conversion steps of the process performed in higher vessels of the stacked series. Thus, most of the deactivating components of the feed stream are removed before it reaches the least contaminated catalyst added to the topmost vessel. However, such systems require valves suitable for closing off catalyst flow against catalyst trapped in the line. Hence, valve life is relatively short and down-time for replacement or repair of the valves is relatively costly.
Alternatively, continuous operation of the hydroprocessing system has included counterflow of catalyst downwardly through a single vessel or a series of vessels in which a hydrocarbon feed stream and hydrogen gas flow upwardly through the catalyst at rates sufficient to ebulate the bed. Such ebulation has been considered desirable or essential to permit withdrawal of catalyst from the lower part of the vessel for slow, but continuous or periodic, removal of catalyst from the vessel. As noted above, such ebulation tends to increase the fluid volume in the vessel relative to catalyst volume necessary to hydroprocess the feed stream and hydrogen with the catalyst, with adequate contact time to react the fluids. Further, such ebulated beds tend to result in separation or segregation of "fines" from the larger (and heavier) particles as they pass downwardly through the upflow streams. As frequently happens, and especially where the catalyst is locally agitated, as by eddy currents, the particles tend to abrade by such higher flow rates of the feed streams through the ebulating bed. Depending on the size of the fines, they either travel upward where they contaminate the product or plug the outlet screen or they tend to accumulate in the reactor because they cannot work their way down to the bottom of the bed. Such counterflow systems have also been used because of the relative ease of withdrawing limited amounts of the ebulated catalyst in a portion of the reacting hydrocarbon and hydrogen fluids, particularly where such turbulent flow of the catalyst is needed to assist gravity drainage through a funnel-shaped opening into a central pipe at the bottom of a vessel.
While it has been proposed heretofore to use plug-flow or packed-bed flow of catalyst to reduce such agitation and thus assure uniform disbursement of hydrogen throughout the liquid volume flowing upwardly through the catalyst bed, in general such flow has been controlled by limiting the maximum flow rate that can be tolerated without ebulating or levitating the bed more than about 10%. Further in prior systems where expansion of the bed is limited, hydrogen flow rates are made sufficiently high at the bottom of the bed to assure relative turbulence of the catalyst at the withdrawal point in the vessel. While this does assure such turbulence, it also wastes space, damages the catalyst and permits direct entrainment of hydrogen with catalyst entering the withdrawal tube. Such turbulent flow of catalyst is apparently necessary to assist gravity removal of catalyst from the vessel.
As particularly distinguished from prior known methods of on-stream catalyst replacement in hydroprocessing, the present method and apparatus provides a system wherein plug flow of the bed is maintained over a wide range of high counterflow rates of a hydrocarbon feed stream and hydrogen gas throughout the volume of the packed catalyst bed. Such packed bed flow maintains substantially maximum volume and density of catalyst within a given vessel's design volume by controlling the size, shape and density of the catalyst so that the bed is not substantially expanded at the design rate of fluid flow therethrough. The proper size, shape and density are determined by measuring bed expansion in a large pilot plant run with hydrocarbon, hydrogen and catalyst at the design pressures and flow velocities as particularly described in Example 2. To further control such packed bed flow, the bed level of catalyst within the vessel is continuously measured, as by gamma ray absorption, to assure that little ebulation of the bed is occurring. Such control is further promoted by evenly distributing both the hydrogen and liquid feed throughout the length of the bed by concentrically distributing both the hydrogen gas component and the hydrocarbon fluid feed component in alternate, concentric annular paths across the full horizontal cross-sectional area of the vessel as they both enter the catalyst bed. Additionally, and as desirable, hydrogen is evenly redistributed and if needed, augmented, through a quench system at one or more intermediate levels along the length of the catalyst bed. Equalizing hydrogen and liquid feed across the full horizontal area along the length of the packed particle bed prevents local turbulence and undesirable vertical segregation of lighter particles from heavier particles flowing in a plug-like manner downwardly through the vessel.
Further in accordance with the method, a system for replacing catalyst during continuing operation of the non-ebulating bed is assisted by carrying out the process at relatively high liquid feed rates, even without ebulation of the bed. In a preferred form, the catalyst transfer system includes an inverted J-tube as the withdrawal tube, so that the tube opens downwardly adjacent the center of the lower end of the vessel and directly above a center portion of the surrounding annular flow paths of liquid and gas into the catalyst bed. Thus the intake for catalyst is out of the direct flow of such streams, and particularly the gas flow. In such a preferred form the annular flow paths are through a conical or pyramidal screen, or perforated plate, which supports the bed or column of catalyst across the vessel through a plurality of radially spaced apart and axially elongated concentric rings, or polygons, supported by radial arms extending from the center of the vessel to the cylindrical side wall of the vessel. Each ring is formed by a pair of peripheral members extending between the radial arms directly under the conical screen so that this forms a circular gas pocket at the upper level in each ring so that between each pair of said peripheral members alternate rings of gas and hydrocarbon liquid enter the bed simultaneously.
In accordance with a further preferred form of the invention, catalyst is both withdrawn from the bed and added to the vessel under laminar flow conditions as a liquid slurry to avoid abrasion and size segregation of particles during such transfer. Both the supply and withdrawal flow lines have a minimum diameter of at least five times and, preferably more than twenty times, the average diameter of the catalyst particles. Further, the flow lines are of uniform diameter throughout their length from either the catalyst supply chamber to the vessel, or from the vessel to the receiving chamber, including the through bore of a rotatable ball of the isolating, pressure control valves, known commercially as "full-port valves". Additionally, in each case a flush line is connected to the flow line between the isolating valve and the reactor vessel so that liquid hydrocarbon may be used to flush the line of catalyst or catalyst fines if necessary, before the valve ball is closed. Preferably, but not necessarily, the withdrawal line may include means for flowing auxiliary hydrogen back into the reactor through the withdrawal tube to prevent coking due to hydrogen starvation near or in the withdrawal tube.
The prior art does not disclose or suggest the above enumerated and pertinent features of either the total system or significant portions of such a system, as disclosed by the following patents:
Jacquin et al. U.S. Pat. No. 4,312,741, is directed toward a method of on-stream catalyst replacement in a hydroprocessing system by controlling the feed of hydrogen gas at one or more levels. Catalyst, as an ebulated bed counterflows through the reactor but is slowed at each of several levels by horizontally constricted areas which increase the hydrogen and hydrocarbon flow rates to sufficiently locally slow downward flow of catalyst. While local recycling thus occurs at each such stage, rapid through-flow of fresh catalyst, with resultant mixing with deactivated or contaminated catalyst, is suppressed. The ebulating bed aids simple gravity withdrawal of catalyst from the vessel. Improvement of the disclosed system over multiple vessels with valves between stages is suggested to avoid the risk of rapid wear and deterioration of valve seals by catalyst abrasion.
Kodera et al. U.S. Pat. No. 3,716,478, discloses low linear velocity of a mixed feed of liquid and H.sub.2 gas to avoid expansion (or contraction) of catalyst bed. By low linear velocity of fluid upflow, gas bubbles are controlled by flow through the packed bed, but the bed is fluidized by forming the bottom with a small cross-sectional area adjacent the withdrawal tube. This assists discharge of catalyst without backmixing of contaminated catalyst with fresh catalyst at the top of the single vessel. The range of the bed level in the vessel is from 0.9 to 1.1 of the allowable bed volume (.+-.10%) due to fluid flow through the bed. A particular limitation of the system is that flow of the fluids undergoing catalytic reaction is restricted to a rate that will not exceed such limits, but must be adequate to ebulate the bed adjacent the catalyst withdrawal tube. Alternatively, injection of auxiliary fluid from a slidable pipe section is required. The patentees particularly specify that the diameter of the lower end of the vessel is smaller to increase turbulence and ebulation of catalyst adjacent the inlet to the catalyst withdrawal line. Fluidization of catalyst is accordingly indicated to be essential to the process. However the disclosed gas flow rates are well below commercial flow rates and there is no suggestion of temperatures or pressures used in the tests or the size, density or shape of the catalyst.
Bischoff et al, U.S. Pat. No. 4,571,326, is directed to apparatus for withdrawing catalyst through the center of a catalyst bed counterflowing to a liquid hydrocarbon and gas feed stream. The system is particularly directed to arrangements for assuring uniform distribution of hydrogen gas with the liquid feed across the cross-sectional area of the bed. Such uniform distribution appears to be created because the bed is ebulating under the disclosed conditions of flow. Accordingly, considerable reactor space is used to initially mix the gas and hydrocarbon liquid feeds in the lower end of the vessel before flowing to other bottom feed distributors. The feeds are further mixed at a higher level by such distributor means in the form of "Sulzer Plates" or a "honeycomb" of hexagonal tubes beneath a truncated, conical, or pyramidal-shaped funnel screen. The arrangement may include an open ramp area parallel to the underside of the screen between the tube or plate ends. Further, to maintain gas distribution along the length of the catalyst bed, quench gas is supplied through upflowing jets in star-shaped or annular headers extending across middle portions of the vessel. The arrangement for withdrawal of spent catalyst requires ebulation of at least the lower portion of the bed. As noted above, added vessel space for uniform mixing of hydrogen and feed before introducing the fluids into an ebulated bed, as well as an ebulating bed, increases the required size of the hydroprocessing vessel, increases catalyst attrition, increases catalyst bed mixing and substantially increases initial, and continuing operating costs of the system.
Bischoff et al. U.S. Pat. No. 4,639,354, more fully describes a method of hydroprocessing, similar to U.S. Pat. No. 4,571,216, wherein similar apparatus obtains uniform ebulation through the vertical height of a catalyst bed, including a quench gas step.
Meaux U.S. Pat. No. 3,336,217, is particularly directed to a catalyst withdrawal method from an ebulating bed reactor. In the system, catalyst accumulating at the bottom of a vessel and supported on a flat bubble-tray may be withdrawn through an inverted J-tube having a particular ratio of the volume of the short leg of the J-tube to the longer leg. The diameter of the J-tube is suited only to flow of catalyst from a body of catalyst ebulated by the upflowing hydrocarbon feed and gas.
U.S. Pat. Nos. 4,444,653 and 4,392,943, both to Euzen, et al., disclose removal systems for catalyst replacement in an ebulating bed. In these patents, the fluid charge including hydrocarbon containing gas is introduced by various arrangements of downwardly directed jets acting laterally against or directly onto the conical upper surface of the bed support screen or screens. Alternatively, the feed is introduced through a conical screen after passing through a distributor arrangement of tortuous paths or a multiplicity of separate tubes to mix the gas and liquid feed over the conical screen. Such arrangements use a considerable volume of the pressure vessel to assure such mixing.
U.S. Pat. Nos. 3,730,880 and 3,880,569, both to Van der Toorn, et al., disclose a series of catalytic reactors wherein catalyst moves downwardly by gravity from vessel to vessel through check valves. As noted above, such valves require opening and closing to regulate the rate of flow, or to start and stop catalyst transfer, with catalyst in the valve flow path. Feed of process fluids is either co-current or countercurrent through the catalyst bed.
Van ZijllLanghaut et al. U.S. Pat. No. 4,259,294, is directed to a system for on-stream catalyst replacement by entrainment of the catalyst in oil pumped as a slurry either to withdraw catalyst from or to supply fresh catalyst to, a reactor vessel. Reacting feed is suggested to be either co-current or countercurrent with catalyst flow through the reactor. Valves capable of closing with catalyst in the line, or after back-flow of slurry oil, are required to seal off the catalyst containing vessel at operating temperatures and pressures from the receiving reactor vessel, or isolate the catalyst receiving lock hopper from the withdrawal section of the vessel.
Carson U.S. Pat. No. 3,470,900, and Sikama U.S. Pat. No. 4,167,474, respectively illustrate multiple single bed reactors and multi-bed reactors in which catalyst is replaced either continuously or periodically. The feed and catalyst flow co-currently and/or radially. Catalyst is regenerated and returned to the reactor, or disposed of. No catalyst withdrawal system is disclosed apart from either the configuration of the internal bed support or the shape of the vessel bottom to assist gravity discharge of catalyst.